This invention relates a system for automatically controlling the cleaning stage of a strip cleaning line, particularly an aluminum strip cleaning line.
In the processing of aluminum strip, e.g. for use in automotive production, it is necessary to clean the surfaces of the strip material. This is typically done by passing the strip material on a continuous basis through a cleaning bath which includes an acid or alkali cleaning section, followed by rinse sections. In each section of the bath, cleaning solution or rinse water respectively is sprayed via nozzles onto the top and bottom faces of the strip passing through the bath. The sprayed liquid flows down into a reservoir in the bottom of the bath from where it is re-circulated by pumps back through the nozzles.
In current aluminum strip cleaning lines, variables which determine the degree of cleaning achieved such as contact time with the cleaning fluid, acid or alkali concentration and bath temperature are not compensated for. At the start of a run, a single target or set-up is used. This means that if one or more variables change during a run or are not at the targeted value, there is no compensating effect from the others. Set-up coils may be used to achieve a steady state in the cleaning bath, but this exercise consumes valuable production time and materials. Typically, in the beginning of a run, the cold coil draws heat from the bath, the bath temperature drops resulting in an under-cleaning condition and the line should slow down to achieve the same degree of strip cleaning. The material not meeting steady state conditions is scrap. An acid or alkali concentration below the target may also result in under-cleaning. If the acid or alkali concentration or the bath temperature is too high or the line speed is too low, over-cleaning may result and the material subjected to these conditions is scrap.
In Sumitomo, JP 11-269678, published Oct. 5, 1990, a cleaning system is described for a continuous steel strip annealing plant. High pressure water jets are used and these water jets are controlled according to the dimensions of the steel strip and the line speed. A control system controls the spray pressure at the center of the strip width as well as at both sides of the strip width such as to adequately wash the entire width of the strip.
Another system is described in Nisshin Seiko, JP 2-290611, published Nov. 30, 1990. This is a system for controlling line speed in a continuous pickling and rolling facility. The system includes loops which are monitored and the objective is to run the pickling equipment at the maximum possible speed and adjust the rolling mill accordingly.
It is the object of the present invention to provide a simplified system for automatically controlling the cleaning stage of an aluminum strip cleaning line.
According to this invention, an automatic control system is provided for the chemical cleaning stage of an aluminum strip cleaning line. In this cleaning line, a chemical cleaning solution, e.g. an acid or alkali cleaning solution, is sprayed onto the top and bottom faces of the aluminum strip as it passes through a cleaning bath. The cleaning solution is recirculated by a pump from a tank or reservoir below the sprays.
A programmable logic controller (PLC) is used and it is supplied with a dwell time set point values for each coil of aluminum strip to be cleaned in the cleaning line. This set point value defines for standard conditions of chemical concentration and temperature the time the strip should be exposed to the cleaning spray. The temperature of the cleaning solution in the reservoir is measured and based on this a signal is sent to the controller. The concentration of the chemical solution in the reservoir is also measured and based on this a further signal is sent to the controller. The temperature and chemical concentration compensated dwell time is then calculated. Based on the compensated dwell time obtained, the dwell time of the cleaning solution spray on the aluminum strip is adjusted such that the coil of aluminum strip being cleaned receives approximately the same degree of cleaning from end to end.
The cleaning solution spray is applied by a plurality of spray nozzles mounted on transverse headers extending across the aluminum strip. The dwell time adjustment is preferably accomplished either by (a) turning on or off flow of cleaning solution to individual transverse headers or (b) having at least some of the transverse headers moveable in the direction of travel of the aluminum strip and moving the headers closer together or farther apart as required. In the extreme case where the required dwell time cannot be provided by method (a) or (b) above, the maximum speed of the line can also be limited to provide the correct dwell time.
The invention compensates for the process variables by increasing or decreasing spray coverage so that the line may continue to run at any speed up to the maximum speed which provides the required cleaning. Thus, when there is a sudden and temporary drop in the cleaning solution temperature and/or a drop in cleaning solution concentration, e.g. when a new coil is started, rather than waiting for the temperature and/or concentration to stabilize, to overcome this temporary aberration, the present invention is used to temporarily compensate for the effect of the temperature drop by increasing the time experienced by the strip in the cleaning solution sprays.
A further feature of the invention comprises an apparatus incorporating a system for moving the transverse headers closer together or farther apart. The apparatus includes carriers or tracks to support the moveable transverse headers, linear actuators for moving the headers and flexible flow connectors for flow connecting the moveable headers. The linear actuators are activated by the programmable logic controller.
A xe2x80x9cDwell Timexe2x80x9d set point value is provided for each per coil to the PLC which defines for standard conditions of acid concentration and temperature, the time the strip should be exposed to the spray. This set point value is downloaded to the process line PLC from a set-up file.
The Line PLC then calculates a temperature and cleaning solution concentration compensated dwell time based on the following algorithm:
Compensated Dwell Time=Dwell Time (set point)+[(Temperature (set point)xe2x88x92Wash Tank Temperature (actual))*Temperature Compensation Factor]+[(Concentration (set point)xe2x88x92Wash Concentration (actual))*Concentration Compensation Factor]
The Compensated Dwell Time value is updated semi-continuously.
The Temperature and Concentration Compensation Factors are pre-set constants in the PLC derived from laboratory generated calibration curves.
A process parameter xe2x80x9cSpray Lengthxe2x80x9d is calculated continually by the PLC during operation based on the following algorithm:
Spray Length=Strip Speed (actual)*Compensated Dwell Time.
The actual dwell time is calculated using the algorithm as follows:
Dwell Time (actual)=Number of sprays on*Average Length of strip covered per spray/Strip Speed (actual)
The maximum permissible line speed which allows constant cleaning conditions can be found as follows:
xe2x80x83Line Speed Limit=Maximum spray length/Compensated Dwell time
Based on the above procedure, the compensated dwell time is maintained by either (a) changing the number of sprays active by turning on/off individual headers or (b) changing the active spray length by moving the spray bars apart or together, and, if necessary, (c) limiting the maximum speed of the line. Any combination of methods a, b, and c may be used. In accomplishing this, line speed, temperature, cleaning solution concentration, spray length and dwell time are all monitored continuously by the line PLC and the appropriate adjustment to the dwell time by the PLC is made so that a correct dwell time is maintained.